1. Field of the Invention
The present invention relates to a rotor for a screw compressor.
2. Related Art
As is known, a screw compressor is equipped with a drive, typically in the shape of a motor, and with a screw compressor element comprising a casing having therein two meshing rotors, whereby one of said rotors, whether or not through a transmission, is driven by the aforementioned drive.
Due to the meshing of the rotors, during the operation of the screw compressor, a fluid, such as air, is sucked in at the inlet of the screw compressor element, which fluid is then compressed between both rotors and is finally expelled at the outlet side of the compressor element under a certain outlet pressure.
The meshing screw shaped parts of the rotors are referred to as the rotor bodies. As is known, one of the rotors has the shape of a male rotor with lobes, while the other rotor has the shape of a female rotor with grooves, in which the lobes of the male rotor mesh in a known manner.
In order to be able to drive the rotors, the rotor bodies are typically provided with a journal on at least one end.
Leakage losses involve a reduction of the efficiency of the screw compressor. To limit these leakage losses, the clearance between the rotors and the clearance between the rotors and the casing of the screw compressor must be kept as small as possible.
Furthermore, in order to prevent damage, any direct contact between the rotor bodies and the casing of the screw compressor is preferably avoided, such that the rotor must not only be strong enough, but also rigid enough.
For this reason, rotors for screw compressors are traditionally manufactured as a single part.
A drawback of this is that material is lost during production.
Another drawback of such single part rotors is that the entire rotor, i.e., both the rotor body and the journals, must be manufactured from the same material.
However, different parts of the rotor pose different requirements to the material to be used.
The possible journals must transmit large forces and must have a very robust bearing.
It is practically impossible to use the journal itself as an inner ring of a bearing. To do so not only necessitates a special type of steel, but also requires a special finishing of the corresponding journal. However, it is not evident to manufacture the entire rotor from such a special type of steel due to reasons of a more difficult processing of such material and the costs involved therein.
The rotor body of a rotor for a screw compressor is preferably made as lightweight as possible. This is desirable because of the high number of revolutions of the rotor during operation of the screw compressor.
Depending on the built-in pressure ratio of the compressor element, the fluid sucked in may heat up strongly during the compression. A fraction of this heat is discharged through the rotor by means of convection. Consequently, the temperature of the rotor may rise very high locally. Also when such relatively high temperatures occur, the strength and the rigidity of the rotors must still be guaranteed.
A material with a low thermal expansion coefficient must be chosen for the rotor body in order to avoid contact with the casing and to reduce leakage losses at the same time.
Another drawback of a single-piece rotor is that it is difficult to provide a suitable cooling channel therein. Although it is possible to provide a central cooling channel through the entire rotor, the cooling efficiency will be limited.
Indeed, the dimensions of the cooling channel may not result in a substantial weakening of the structure. This results in the distance between the cooling channel introduced and the outer surface of the rotor becoming too large to obtain efficient cooling.
Yet another drawback is that it is difficult or even impossible to repair a rotor when only a single part, such as the journal or the rotor body, is damaged.
It is also disadvantageous that placing sensors in the rotor, for example for measuring vibrations or temperature, is difficult.
From the foregoing, it is clear that single-piece rotors for screw compressors have a series of drawbacks.
Therefore, the present invention aims to offer a solution for at least one of said and/or other drawbacks.